Thermoplastic polymeric materials are foamed to provide low density articles such as films, cups, food trays, decorative ribbons, and furniture parts. For example, polystyrene beads containing low boiling hydrocarbons such as pentane are formed into lightweight foamed cups for hot drinks such as coffee, tea, hot chocolate, and the like. Polypropylene can be extruded in the presence of blowing agents such as nitrogen or carbon dioxide gas to provide decorative films and ribbons for package wrappings. Also, polypropylene can be injection molded in the presence of blowing agents to form lightweight furniture parts such as table legs and to form lightweight chairs.
Polyesters such as poly(ethylene terephthalate) typically have a much higher density (e.g. about 1.3 g/cc) than other polymers. Therefore, it would be desirable to be able to foam polyester materials to decrease the weight of molded parts, films, sheets, food trays, and the like. Such foamed articles also have better insulating properties than unfoamed parts. However, it is difficult to foam such polyester materials because of the low melt viscosity and low melt strength of typical poly(ethylene terephthalate) and related polyester polymers. The low melt viscosity and low melt strength of the polyesters is a problem because the polymer melt will not adequately retain the bubbles of an expanding gas. It would be desirable therefore to be able to provide polymers which could be foamed with conventional foaming systems.
One approach to providing polyesters having high melt strengths and melt viscosities involves treating preformed polyesters with multifunctional carboxylic acids or polyols to provide branched polyesters. Such compositions are disclosed in U.S. Pat. Nos. 4,132,707; 4,145,466; 4,999,388; 5,000,991; 5,110,844; 5,128,383; and 5,134,028. The branching agents used include tri- and tetracarboxylic acids and anhydrides such as trimesic acid, pyromellitic acid, and pyromellitic dianhydride or polyols such as trimethylolpropane and pentaerythritol.
Polyesters containing dicarboxylic acid sulfomonomers are disclosed in U.S. Pat. Nos. 3,734,874; 3,779,993; 4,335,220; 4,233,196; 3,853,820; and 5,053,482. U.S. Pat. Nos. 3,734,874 and 4,233,196 are concerned with amorphous water dispersible polyesters which contain at least 8 mole percent of a sulfomonomer and substantial amounts of a difunctional glycol ether component. U.S. Pat. No. 3,853,820 describes an amorphous water dissipatable polyester with at least 20 mole percent of a poly(ethylene glycol) which is a condensation polymer of ethylene glycol. In addition, U.S. Pat. No. 5,053,482 describes fiber- and film-forming polyesters based on polyethylene terephthalate polyesters containing 20 to 40 mole percent of diethylene glycol for use in disposable products such as disposable diapers. U.S. Pat. Nos. 4,499,262; and 4,579,936 pertain to polyester compositions useful for the manufacture of bottles.
U.S. Pat. No. 5,399,595 discloses polyesters which contain small amounts of residues of a dicarboxylic acid sulfonate monomer containing monovalent metal ions, e.g., alkali metals, and foamed articles prepared therefrom. According to the patent, the presence of an alkali metal sulfonate monomer, e.g., residues of 5-sodiosulfoisophthalic acid, in the polyesters provides the copolyester compositions with increased melt viscosities.